Weil McLain F39

What does Weil McLain F39 mean?

F39 indicates the outdoor temperature sensor (used for outdoor temperature compensation / OTC) has gone outside its normal operating range of −22°F to +158°F, caused by a short circuit or open circuit condition. If the sensor open-circuits and an OTC curve is selected, the OTC function is disabled but central heating continues without compensation. A short circuit sustained for more than 10 seconds generates the fault.

Symptoms

• F39 displayed on the control module LCD.
• Outdoor temperature compensation curve is disabled; boiler runs at fixed setpoint.
• Fault may clear automatically when sensor reading returns to the valid range.
• A16 may display instead of F39 on heating-only boilers if the sensor is completely absent.

Common causes

• Outdoor sensor wiring short-circuited from physical damage, moisture, or pinching.
• Outdoor sensor disconnected from the boiler after OTC was active.
• Sensor failed — open circuit due to age or water ingress.
• Corroded sensor connector terminals.

Diagnostic steps

1. 01

   Inspect outdoor sensor wiring from sensor to boiler

   Trace the sensor wiring along its full run. Look for cuts, pinching by door or window frames, or sections exposed to moisture. Repair any damaged sections.

2. 02

   Check the sensor connector at the boiler

   Unplug and re-seat the outdoor sensor connector at the control board. Inspect for corrosion or bent pins and clean terminals if necessary.

3. 03

   Measure sensor resistance

   Disconnect the sensor and measure its resistance at the current outdoor temperature. Compare against the sensor resistance table in the boiler manual. An open circuit (infinite resistance) or short (near zero) confirms a failed sensor.

4. 04

   Replace the outdoor sensor if defective

   Install the correct Weil-McLain outdoor sensor. Route and secure the new sensor wiring to protect it from physical damage. Restore power and reset the boiler.

5. 05

   Disable OTC if sensor is not needed

   If the installation is exempt from OTC requirements, access the OTC Exempt parameter and set it to 1. This prevents F39 from appearing when no outdoor sensor is connected.

When to call a professional

Call a licensed technician if F39 persists after sensor replacement, or if the wiring run requires re-routing through walls or exterior building penetrations. A technician should also verify OTC exempt parameter settings if the installation is qualified to operate without an outdoor sensor under the applicable energy code requirements.